Glossy reflection using Principled BSDF is much rougher, why?
Too much Glossy Filtering

As said in the manual:


  When using a value higher than 0.0, this will blur glossy reflections after blurry bounces, to reduce noise at the cost of accuracy. 1.0 is a good starting value to tweak.


You are using a value of 4.0, that means you are strongly using the filtering for glossy bounces.



Use a lower values to lower the strenght of the blur applied.



That said, actually the principled glossy shader behaves quite differently in comparison with the mere glossy shader in the sensitivity of the filtering.